1. Field of the Invention
The present invention relates to an improved focus detecting device for use in a photographic camera, and more particularly to an improved device adapted to automatically detect a focusing status of the optical imaging system as incorporated in a photographic image taking apparatus such as a single-lens reflex camera and the like.
2. Description of the Prior Art
Heretofore, there has been proposed the use of a pair of light-receiving or light-sensing element arrays for the detection of a state of focusing such as a proper focusing, a front focusing or a rear-focusing of the optical imaging system as incorporated in a photographic camera and the like in the application of a device for detecting a focusing state thereof. In such an application, it is generally known that there is essentially required a certain means of splitting or dividing a beam of light which passes through the image forming lens system of a photographic camera so that the thus-split beam or beams of light may be directed upon a series of light sensing elements.
Now, referring to FIG. 1, there is shown a typical example of such an arrangement which is adapted to detect a proper focusing position of an optical image as obtained in a common optical apparatus by way of an extent of sharpness thereof. FIG. 1 is a schematic view in cross-section taken along a plane which is at a right-angle with respect to a pair of light-sensing element arrays 6 and 7, according to this particular arrangement, there are a series of light-sensing elements, which are arranged in paired first and second arrays 6 and 7 extending in parallel with and at a constant given distance from each other along the optical axis of an image-forming lens 1. The elements are arranged to detect information from a pair of optical images; received from each of the pair of light-sensing arrays so as to obtain the extent of sharpness of the thus-obtained optical information. Sharpness data of each image of the split beams of light from the light-sensing element arrays are compared with each other in accordance with a predetermined evaluation function, thereby to determine which array of light-sensing elements carries data nearer a current position of proper focusing through the optical system. As typically shown in FIG. 1, there is provided an optical beam splitting prism 4, which is adapted to specifically split an incident beam of light from the imaging lens 1 in such a manner that the thus-split beams of light may individually be directed upon the pair of light-sensing element arrays 6 and 7 with a given differential path of light. In practice, it is essential that the split beams of light have properties which are as identical as possible except for their differential paths or routes of travel through the beam-splitting prism. In this respect, it is therefore essential to use a particular caution in the preparation of a semi-permeable membrane 8 for the purpose of splitting a beam of light. More specifically, this particular semi-permeable membrane should necessarily be prepared such that it presents a typical light-splitting ratio of 1 or so, while having an as small as possible light absorption factor to attain an optimal photoconductivity. Since the scene or object to be taken is generally illuminated with white light, it is undesirable to have a possible deviation in the ratio of light-splitting in accordance with the wave length of light. In this connection, when this semi-permeable membrane is made of a metal with deliberate selection of an appropriate material as well as of due conditions in the manufacturing process, it may be attained readily with desired properties, except for an inherent drawback of a substantial loss of light which is as great as 50% or so. While it is expectable to realize a substantial reduction in loss of light with the adoption of a dielectric substance as the material for a semi-permeable membrane in practice, it would undesirably result in a marked extent of variation in the ratios of light splitting with changes in the wave length of light, because of its light interference effect. In addition, there is an inevitable problem of polarization of light which is also intrinsic thereto. That is, to be more strict, it is noted that there is generally observed a certain extent of polarization, more or less, in light which is directed into the image forming or objective lens 1 from an object. Also, since the light incident through the lens 1 is generally then introduced to a light detecting unit 5 through such routing means as a semi-transparent mirror 2 and a reflection mirror 3, the current state of polarization of the light is influenced by the specific optical characteristics of the semi-transparent mirror 2, the reflection mirror 3, etc. Incidentally, since the ratio of light splitting specific to the semi-permeable membrane 8 would practicably be influenced to a substantial extent, by the specific condition of polarization to the then incident light thereto, it is difficult to practice to hold a constant rate of light splitting at substantially 1, unless the current state of polarization of light incident upon the beam-splitting prism 4 is made stable. In this respect, therefore, it would become essential for such construction to provide a suitable polarizer 10 in front of the beam-splitting prism 4, as schematically shown in FIG. 1.
With the employment of such an additional polarizer to the construction of the focusing detection device, however, it is naturally inevitable that there would occur an added loss of light to a substantial extent, as a consequence.
In this respect, the present invention is essentially directed to meet such inconveniences and difficulties as was encountered in the conventional focusing state detecting device for use in a photographic camera or the like which have not been attended with any due countermeasures therefor.